Field of the Invention
This invention relates to communications systems and more particularly to managing interference associated with communications systems.
Description of the Related Art
In a typical radio frequency (RF) communications system, a transmitter generates a relatively high-power radio-frequency signal carrying information received from an input data signal. A typical RF transmitter includes a modulator, an RF mixer (i.e., RF modulator), and a power amplifier. The modulator converts digital data from the input signal into an analog waveform, typically a baseband signal, although not limited thereto. The RF mixer translates the center frequency of the baseband signal to a target RF frequency. The power amplifier boosts the power of the signal for transmission by an antenna over the channel (e.g., over-the-air).
In practice, the typical transmitter does not produce a completely bandlimited signal. As referred to herein, a “bandlimited” signal is a signal that has energy only in a target frequency band of the channel. In general, the target frequency band and other frequency bands are separated by a guard band, which in typical orthogonal frequency division multiplexing (OFDM) transmitters is included as part of the target frequency band, but is not allocated any data. Energy that is transmitted outside the target frequency band is referred to as “out-of-band” energy. In general, out-of-band energy causes interference with other channels (e.g., adjacent channels or alternate adjacent channels) and limits the performance of other communications systems operating in proximity to the radio-frequency communications system. To manage the amount of out-of-band energy and interference between proximate communications systems, communications standards define spectral masks that transmitted signals must satisfy to be compliant with the communications standard. A typical communications standard for RF communications systems specifies a spectral mask that defines characteristics of in-band signals for a target communications channel based on a frequency band having a particular center frequency and a particular bandwidth. The spectral mask also defines characteristics for out-of-band signals (e.g., how energy transmitted outside the defined communications channel is to be attenuated as a function of frequency).
In actual communications systems, the modulator, RF mixer, and power amplifier each introduce out-of-band energy. However, the power amplifier is typically the limiting factor in the RF transmitter. In general, tolerable levels of distortion, which are specified by an applicable communications standard, determine the maximum output power deliverable by the power amplifier. Manufacturing tolerances cause power amplifiers to have different gains and different levels of distortion at the same output power level. In addition, during transmitter operation, gain and distortion levels may change in response to increase in temperature due to power dissipation of the transmitter. Accordingly, the transmitter of an actual communications system is not typically configured to operate at a maximum output power at which the target distortion levels are just reached. Instead, the transmitter may be configured to generate transmit signals having a power level that has substantial margin below the distortion limits. For example, the output power may be set at 5 decibels (dB) or more below a maximum transmitter output power at which the target distortion levels are just reached. Another technique for setting the output power of a transmitter includes calibrating each individual transmitter, which introduces additional costs into the end product. For example, calibration requires time to obtain calibration data and non-volatile memory to store the calibration data. In addition, calibration techniques that account for temperature variation are limited.